1. Field of the Invention
The present invention relates to a coil component. In particular, the present invention relates to a coil component including a planar spiral conductor formed on a printed-circuit board by electrolytic plating and a method for manufacturing the same.
2. Description of Related Art
In the field of consumer and industrial electronic devices, surface-mounting type coil components have been used more and more as power supply inductors. The reason is that surface-mounting type coil components are small and thin, provide excellent electrical insulation, and can be manufactured at low cost.
Among specific structures of a surface-mounting type coil component is a planar coil structure using printed-circuit board technology (for example, see Japanese Patent No. 4873049). The structure will be briefly described in terms of manufacturing steps. Initially, a seed layer (base film) having a planar spiral shape is formed on a printed-circuit board. The printed-circuit board is then immersed into a plating solution and a direct current (hereinafter, referred to as “plating current”) is passed through the seed layer, whereby metal ions in the plating solution are electrodeposited on the seed layer. This forms a planar spiral conductor. An insulating resin layer which covers the formed planar spiral conductor and a metal magnetic powder-containing resin layer which serves as a protective layer and a magnetic path are then formed in succession to complete the coil component. Such a structure can maintain dimensional and positional accuracies at extremely high values, and allows a reduction in size and thickness. Japanese Patent Application Laid-Open No. 2006-66830 discloses a planar coil element having such a planar coil structure.
The purpose of using the foregoing electrolytic plating for the formation of the conductor is to make the conductor thickness of the planar spiral conductor as large as possible. The applicants then perform special plating that the applicants call “HAP” (High Aspect Plating) to allow a further increase in the conductor thickness.
HAP uses a current higher than heretofore for electrolytic plating to quickly grow a plating layer of electrodeposited metal ions. This can provide a thicker plating layer than theretofore, whereby the conductor thickness of the planar spiral conductor can be made greater than heretofore.
However, HAP can sometimes cause an abnormal lateral growth of the plating layer at a portion corresponding to the outermost turn of the planar spiral conductor. More specifically, in HAP, the high plating current tends to grow the plating layer in lateral directions. If there is any other adjoining seed layer, the lateral growth is suppressed by the presence of the plating layer growing on the other adjoining seed layer. On the other hand, if there is no other adjoining seed layer like the outermost turn of the planar spiral conductor, nothing suppresses the lateral growth. The outermost turn therefore becomes excessively large in line width, causing the problem that a desired spiral pattern cannot be formed. The lateral growth needs to be prevented in particular because such a pattern can deteriorate the characteristics of the coil component.
To meet a demand for high-density mounting, it is needed to reduce the area occupied by the coil component while securing a desired mounting strength of the coil component. In particular, it is needed to secure a desired mounting strength with a minimum amount of solder for reduced material cost.
For high-density mounting, an external electrode structure where electrode surfaces are formed only on the chip bottom has been increasingly used recently. The omission of electrode surfaces from the side surfaces of a chip precludes the formation of solder fillets, whereby the area occupied by the chip component can be reduced. If a coil component has the electrode structure including only bottom electrodes, the ends of the planar spiral conductor need to be led out to the bottom side of the chip component and connected to the external electrodes instead of being led out to lateral sides of the chip. This needs some contrivance to the electrode structure. In particular, the bottom electrodes need to have a sufficient area to provide a bonding strength at the time of surface mounting.